The invention relates to a low-pressure mercury vapour discharge lamp provided with a tubular discharge vessel with an internal diameter D which encloses a discharge space, which contains mercury and a rare gas, in a gaslight manner and in whose end portions electrodes are arranged, said discharge vessel being provided at an inner surface with a luminescent layer which supports a protective layer comprising a metal oxide.
The invention also relates to a method of manufacturing the low-pressure mercury vapour discharge lamp whereby the discharge vessel under construction is provided with the luminescent layer at the inner surface, after which a suspension comprising a metal oxide is provided on the luminescent layer.
Such a lamp is known from DE 32 01 606 C1. The known lamp has a protective layer of aluminium oxide over a luminescent layer of halophosphates. Owing to the presence of the protective layer, the light output of the lamp decreases less strongly in the operating period after 100 hours of operation than is the case with a lamp without protective layer.
The luminescent layer is provided in the known lamp in that a suspension of luminescent material is caused to flow from an end over the inner surface of the discharge vessel until it issues from the other end of the discharge vessel. The provision of the luminescent layer in the form of a suspension was found to be a convenient method which is widely used. The protective layer is provided on the luminescent layer in an analogous manner. The protective layer is not provided, however, until after the luminescent layer has been sintered, i.e. heated for some time so as to drive out from the layer the auxiliary substances still present in the layer. Sintering of the luminescent layer before the protective layer is provided thereon is necessary in order to avoid luminescent material being washed away when the metal oxide suspension flows over the surface thereof. After the provision of the protective layer, however, a second sintering operation is necessary for removing the auxiliary substances from this layer also. The repetition of this comparatively time-consuming operation renders the manufacture of the known lamp comparatively inefficient compared with that of a lamp without protective layer.